1. Field of the Invention
This invention relates to a system for controlling valves, and more particularly to an automatic system for controlling the valve on a prover loop during a test.
2. Description of the Prior Art
The prover loop system for which this invention was designed is usually connected in series with a pipeline from a LACT unit, downstream from the flowmeter in the LACT unit. The prover loop typically has a control valve which reverses the flow through the prover loop. Inside the prover loop is a flexible sphere which is pushed through the prover loop by the flow of oil or other hydrocarbon. Two sphere detectors are located at known distances apart on the inside of the prover loop. The time it takes for the sphere to travel between the two sphere detectors is determined and used to calculate the flow rate, thereby calibrating the LACT unit flow meter. It is customary in the industry to use one "round trip" to complete one test. Three consecutive test results within a certain tolerance are required before a flow meter's accuracy can be certified. Industry standards require these flow meters to be calibrated once a month.
By necessity, the prover loop calibration must be precisely controlled in order to obtain accurate results. Heretofore, prover loop controls have employed a hydraulic system for operating the prover loop valve which reverses the flow direction within the prover loop. These hydraulic systems are expensive and usually require at least a 1 K.V.A. transformer to meet the necessary power requirements. Also, the hydraulic systems require constant operator attention to achieve the three consecutive test results within the required tolerance. The operator must use a stop-watch so the same amount of liquid flows through the LACT unit flow meter each time the valve is reversed. Fractions of a second can greatly affect the test results. Extreme temperatures also affect the viscosity of the liquid, thereby varying the time it takes to change valve positions. Because of these inaccuracies, the prover loop operator can often waste much time trying to obtain the three consecutive tests within the given tolerance.